The present invention relates to dual flowtube Coriolis mass flowmeters which comprise two brace bars that are connected to the flowtubes to decouple certain extraneous modes of vibration from the desired driven and Coriolis modes of vibration of the flowtubes. More particularly, the invention relates to such a flowmeter wherein the brace bars are oriented on the flowtubes such that the reaction forces at the boundary conditions of the flowmeter are effectively reduced or eliminated, thereby improving the zero stability of the flowmeter.
Dual flowtube Coriolis mass flowmeters commonly comprise two identical flowtubes, an inlet manifold for connecting a first end of each flowtube to a first section of process piping through which a fluid to be measured flows, an outlet manifold for connecting a second end of each flowtube to a second section of the process piping, one or more force drivers for vibrating the flowtubes in one of their natural modes of vibration, such as the first bending mode of vibration, and a number of motion sensors for detecting the vibrating motion of the flowtubes. The flowtubes can have various configurations, such as S-shaped and U-shaped, and each flowtube lies in a plane that is parallel to the plane of the other flowtube. Furthermore, the force drivers are mounted so as to vibrate the flowtubes toward and away from each other in a direction which is perpendicular to the planes of the flowtubes.
As the fluid flows through the vibrating flowtubes it generates Coriolis forces that cause the flowtubes to deform into a unique and characteristic shape, which is commonly called the xe2x80x9cCoriolis deflectionxe2x80x9d. Thus, in operation the flowtubes are subject to a xe2x80x9cdrivenxe2x80x9d mode of vibration, which is generated by the force drivers, and a xe2x80x9cCoriolisxe2x80x9d mode of vibration, which results from the Coriolis forces acting on the flowtubes. As is well understood by those of ordinary skill in the art, the mass flow rate and certain other properties of the fluid can be determined from the Coriolis deflections of the flowtubes, which are measured by the motion sensors.
One problem with many prior art Coriolis mass flowmeters is their inability to maintain a stable xe2x80x9czeroxe2x80x9d signal, that is, the output of the meter in the presence of zero flow. In contrast to Coriolis mass flowmeters, mechanical positive displacement type flowmeters have an inherent zero. This results from the fact that, when fluid stops flowing through these meters, the flow detection mechanism also stops moving, thus allowing for the measurement of an absolute zero flow condition. Coriolis mass flowmeters on the other hand have a xe2x80x9clive zeroxe2x80x9d. This is due to the fact that the motion sensors are inherently incapable of determining whether the flowtube deflections are caused by the force drivers alone or by the Coriolis forces generated by fluid flowing through the vibrating flowtubes. Therefore, during installation and startup of the Coriolis mass flowmeter, the operator must usually shut off the flow through the meter and initiate a zeroing procedure, which essentially tells the meter that the measured flowtube deflections correspond to a zero flow condition.
However, the zero signal in certain prior art Coriolis mass flowmeters may be sensitive to changes in ambient conditions, such as the temperature and pressure of the fluid, external stresses on the flowtubes and extraneous vibrations on the process piping. These changes can cause the zero signal to xe2x80x9cdriftxe2x80x9d, which results in the meter indicating some amount of flow when in fact no flow exists, a condition which is often called xe2x80x9czero shiftxe2x80x9d. A major cause of this problem is imbalance in the vibrating system of the meter, which primarily comprises the flowtubes and their attached hardware. This imbalance allows the vibrational energy to be transmitted out of the vibrating system and into the boundary conditions of the meter, which are generally taken to be the sections of the process piping to which the meter is connected. This vibrational energy will then be absorbed or reflected by the boundary conditions and, depending on the amplitude and phase of the absorbed or reflected energy, can cause a zero shift in the output of the flowmeter.
Brace bars are commonly used on dual flowtube Coriolis mass flowmeters to link the flowtubes together near where they connect to the inlet and outlet manifolds. In this manner, the brace bars serve to couple the vibrational energy of each flowtube to the other in order to keep the flowtubes vibrating in opposition to each other and thereby maintain the vibrating system in balance. Each brace bar is typically a flat metal plate having two transverse openings through which the flowtubes are received and secured, such as by brazing or welding. In addition, each brace bar is normally mounted such that the plane of the brace bar is generally perpendicular to the axes of the flowtubes. Consequently, the principal axes of the brace bar are also perpendicular to the axes of the flowtubes.
However, when the brace bars are mounted to the flowtubes in this fashion, their principal axes are normally not aligned with the effective mass centers of the flowtubes. As will be explained more fully below, the effective mass centers are the locations in space of four point masses of an equivalent mass-spring system which may be used to represent the vibrating system of the flowmeter. The misalignment between the principal axes of the brace bars and the effective mass centers of the corresponding halves of the flowtubes to which they are attached can cause the brace bars to deflect out of plane when the flowtubes are vibrated in the driven and Coriolis modes. These deflections can in turn create undesired reaction forces which may be transmitted to the boundary conditions of the flowmeter and thereby cause a zero shift in the output of the flowmeter.
In accordance with the present invention, these and other problems in the prior art are overcome by providing a Coriolis mass flowmeter which comprises at least first and second generally parallel flowtubes, each of which includes a first half that is connected to an inlet manifold and a second half that is connected to an outlet manifold, a first brace bar which is attached to the first halves of the flowtubes, and a second brace bar which is attached to the second halves of the flowtubes. Furthermore, the first and second brace bars are oriented on the flowtubes such that, when the flowtubes are vibrated in at least one of a driven mode of vibration and a Coriolis mode of vibration, the resulting reaction forces at the inlet and outlet manifolds are less than those that exist when the first and second brace bars are oriented generally perpendicular to the flowtubes. In a preferred embodiment of the invention, the first and second brace bars are oriented on the flowtubes such that, when the flowtubes are vibrated in at least one of a driven mode of vibration and a Coriolis mode of vibration, the resulting reaction forces on the inlet and outlet manifolds are approximately zero.
When oriented in this fashion, the brace bars will be subjected to little or no out of plane deflections from the driven and Coriolis modes of vibration of the flowtubes. Consequently, the reaction forces on the boundary conditions will be minimized or eliminated. Therefore, the detrimental effects that these reaction forces have on the zero stability of the flowmeter will likewise be minimized or reduced.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.